Various types of fuels, but most notably fuel oils, are frequently transferred from one marine vessel to another, as well as between floating vessels and shore installations. The transfer is accomplished in most, if not all, cases using cargo hose which has an inner diameter of several inches (various sizes being used for different circumstances) and can accomodate quite high flow rates to thereby keep the fuel transfer time to a minimum. The hose can, and often is, supplied in sections, each of which can be 25 or more feet long, and the sections can be coupled together to form a hose having the needed length.
As will be recognized, with a hose several inches in diameter and twenty or more feet long, a sizable quantity of fuel can exist in the hose itself at any time. After transfer of fuel to or from a tank vessel, it is common practice to attach a closure to the output end of the hose, this closure being a metal disc, called a blank, having holes so that it can be bolted to a fitting on the hose end. Attachment of the blank prevents fuel contained in the hose from spilling out at that time, but ultimately it is necessary to remove the blank and, unless that is done very carefully, fuel is spilled.
Simple calculation shows that an 8" ID hose section 25 feet long can contain over 65 gallons of oil. Thus, the quantity involved in accidentally dumping a hoseful of oil is by no means trivial, and spills such as that can have a serious detrimental ecological effect as well as being wasteful of this increasingly precious commodity.